1. Field of the Invention
The present invention relates to a random number generating method and a random number generating device, and more particularly to a method and a device for generating natural random numbers or physical random numbers based on a thermal noise fed from a thermal noise generating source.
The present application claims priority of Japanese Patent Application No.2001-272538 filed on Sep. 7, 2001, which is hereby incorporated by reference.
2. Description of the Related Art
A variety of devices or methods of generating random numbers by using random events detected in the natural world are conventionally proposed. As a device of such kind, a random number generator by using, for example, a radioactive decay of an atomic nucleus is available. However, since there is a problem being peculiar to such the device handling radioactive rays, in current popular use is a use of a random number generator using a thermal noise produced by a resistor, diode, or a like.
As an example of a device to generate random numbers using a thermal noise, a random number generator is disclosed in Japanese Patent Application Laid-open No. 2001-134422. FIG. 9 is a schematic block diagram showing configurations of a conventional thermal noise physical random number generator disclosed in Japanese Patent Application Laid-open No. 2001-134422. In the thermal noise physical random number generator 100, a thermal noise voltage generated from a thermal noise generating device 101 such as a resistor, diode, or a like is amplified by a preamplifier 102 and a main amplifier 103 and a crest value of an analog signal obtained by amplification is selected by a pulse height discriminator 104 according to a threshold value.
A waveform of a pulse thus selected is shaped by a waveform shaper 105 so as to become a rectangular wave signal and the waveform-shaped rectangular wave signal is transmitted to a time measuring device 106. The time measuring device 106 measures a time interval between pulses, based on a clock pulse generated by a time measuring pulse generator 107 and counts a number of pulses in a specified bit unit. A random number generator 108 takes out a counted number of bits as random numbers which are then stored in a random number storing device and controller 109.
When an amplifier being used in such the random number generator 108 as described above (for example, the above preamplifier 102, and the main amplifier 103) performs ideal amplification, if each instantaneous value of an input is independent from one another, an instantaneous value of its output is also independent. Therefore, when a pulse is produced by comparing the instantaneous value with a predetermined voltage, its pulse interval is provided in an ideal manner. If the predetermined voltage is sufficiently higher than a root mean square (rms) value of a voltage value obtained after the amplification, a probability of occurrence of a pulse to be produced by comparing with amplified thermal noise follows a Poisson distribution and the time interval between pulses follows exponential distribution. Then, when the time interval is measured by an N-bit counter, the counted values become uniform random numbers if occurrence frequency of a pulse, clock frequency of a counter, numbers of bits of the counter or a like can satisfy a specified condition.
However, in an actual amplifier or amplifying circuit, it is unavoidable that its response time is finite and its band is also finite. As a result, even if each instantaneous value of an input is independent, each instantaneous value of its output is not independent. Therefore, when a pulse is produced by comparing a voltage obtained after amplification with a specified voltage, since the comparison is made by a voltage value affected by its previous voltage value, the time interval between pulses does not follow exponential distribution and invalid pulses are mixed in addition to pulses following original exponential distribution.
FIG. 10A is a diagram showing a waveform of a signal obtained after amplification in the case where an amplifier provides an ideal behavior in which a crest of a voltage having a small amplitude occurs immediately after a crest of a voltage having a large amplitude. When a waveform of the signal having the crest of the voltage with the large amplitude is compared with the signal having the crest of the voltage with the small amplitude by using a threshold value 201 shown by a dashed line in FIG. 10A serving as a reference voltage, a pulse 202 is produced only by the crest of the voltage having the large amplitude as shown in FIG. 10B.
However, in an actual amplifier, if a previous voltage value is mixed in the voltage obtained after the amplification, as shown in FIG. 11A, a crest of the voltage occurring immediately after the crest of the voltage having the large amplitude becomes higher when compared with the case of an amplifier which provides an ideal behavior. Therefore, when the amplitudes of the signal having the two crests are compared by using the threshold value 205 shown by a dashed line serving as a reference value, though the crest of the voltage having a small amplitude is lower than the crest of the voltage having the large amplitude at a time of signal inputting, the former is involved in the generation of a pulse and, as shown in FIG. 11B, an invalid pulse 207 in addition to an originally desired pulse 206 is produced.
As described above, when random pulses are generated using the waveform of the signal shown in FIG. 11(a), since the invalid pulse 207 described above is mixed in with desired pulses 206 to be originally produced, uniformity of random numbers is disturbed. That is, in the conventional thermal noise physical random number generator 100, production of random numbers is affected by a response characteristic of the amplifiers 102 and 103 because the random numbers are produced by using pulses in which invalid pulses 207 are mixed. This presents a problem in that, since random numbers having small random number codes occur more than those having other random number codes, uniformity of output random numbers cannot be secured.
As a result, a secondary problem is produced in that a bias existing in the occurring random number codes causes a critical defect of a random number generator and such the random number generator, when being mounted on other devices requiring a true random number, cannot provide a desired result.